In the PS-SE . . . (patent application No. 8403529-4), which is a priority application upon which applicant's co-pending U.S. application Ser. No. 747,830, filed June 24, 1985, and replaced by pending U.S. application Ser. No. 120,782, is based, a method and an apparatus are disclosed, which are used for the heating of thick-walled glass tubes at the manufacture of optical fibres. According to said patent the tube is heated by means of microwave energy.
The method according to the said patent is especially characterized in that the tube is pre-heated to a temperature of about 1000.degree. C.-1500.degree. C., preferably by means of a gas flame in a manner known per se, whereafter the tube is heated by means of microwave energy generated by a microwave generator, by introducing the tube axially into a microwave cavity comprising in its two end walls openings for the tube, and the electrical field strength is given a field image including only one tangential component, according to TE-01n-mode, preferably according to the mode TE-011, whereby the electrical field is formed so as to be tangential to the surfaces of the tube, and formed so that the electrical field strength is zero adjacent the surfaces of the cavity.
According to a preferred embodiment of said patent the microwave cavity is of cylindrical configuration and made of metal, and its two end walls include, as mentioned, openings, through which the tube to be heated is axially introduced.
The cavity described in said Swedish patent involves the problem of offering relatively limited possibility of supplying the cavity with such power, that a high temperature of the tube is obtained without thereby causing electrical arcing.
This limitation partly is due to the fact that the thermal dissipation from the heated tube is high, and partly because at the passing of the cavity relative to the tube the hot portion of the tube is moved to one end of the cavity, whereby the electrical field is utilized for heating the tube already heated to an extent greater than desired. It is hereby difficult to supply sufficiently high power to the centre of the cavity, i.e. where the tube portion to be heated is located, without giving rise to electric arcing.
By using the said TE-01n-mode, the cavity is given a relatively high so-called Q-value. Considering that the maximum power to be supplied is limited by the necessity of preventing electric arcing, it is desired to be able to reduce the Q-value of the cavity and thereby to be able to supply higher power without causing electric arcing.
The said TE-011-mode when used in a cylindric cavity yields a distribution, which is highly advantageous at the heating of quartz tubes to collapsing temperature. The electrical field is zero at the envelope surface of the cavity and in the cavity centre, while the field strength is high in the walls of a thick-walled glass tube.
At the manufacture of optical fibres, the collapsing is preceded by a deposition phase and a sintering phase.
On the inner wall of the glass tube layers of pure quartz SiO.sub.2 are deposited. In certain layers also germanium dioxide GeO.sub.2 is deposited as a doping agent. This is effected in that SiCl.sub.4 and oxygen O.sub.2, together with GeCl.sub.4, are introduced into the tube. SiO.sub.2 and GeO.sub.2 are herewith deposited on the inner surface of the tube. In order for these reactions to take place, it is necessary to heat the interior of the tube to about 1400.degree. C. This is normally carried out by passing a burner along the entire length of the tube, while rotating the tube about its axis. Firstly, SiCl.sub.4 and O.sub.2 are introduced, whereupon SiO.sub.2 is deposited and forms a porous layer. When the burner passes the porous layer, the layer sinters to form a transparent SiO.sub.2. This deposition is repeated, whereafter GeCl.sub.4 is mixed with the SiCl.sub.4 gas, so as to develop a correct so-called index profile. Normally, 30 to 100 layers are applied during the deposition phase. After the deposition phase and sintering phase, the temperature of the burner is raised so as to heat the tube to about 2200.degree. C. At this temperature the surface tension causes the tube to draw together, i.e. to collapse. After the burner has been traversed a few times, the tube forms a rod, i.e. a so-called preform, from which an optical fibre can be drawn.
During the deposition phase and sintering phase a sufficiently high temperature is desired to prevail on the inside of the tube wall. As during the collapsing phase, the problem exists also during these phases that the maximum supplied power is limited by the necessity of preventing electric arcing.